Heterojunction biopolar mixer circuitry

ABSTRACT

Mixer circuitry having a semiconductor body formed therein mixer circuitry having an oscillator having a heterojunction bipolar transistor and a mixer having a Schottky diode. The heterojunction transistor has a collector region formed in one portion of doped layer of the semiconductor body and the diode has a metal electrode is Schottky contact with another portion of such doped layer. The mixer is includes a diode and a DC biasing circuit, comprising a constant current, for biasing such diode to predetermined operating point substantially invariant with power of an input signal fed to such mixer.

This is a divisional of U.S. patent application Ser. No. 08/774,237,filed Dec. 27, 1996, now U.S. Pat. No. 5,837,589.

BACKGROUND OF THE INVENTION

This invention relates generally to mixer circuitry and moreparticularly to mixer circuitry having a local oscillator and mixerformed as a monolithic microwave integrated circuit (MMIC).

As is known in the art, mixer circuitry is used in a wide variety ofapplications. In one such application, the mixer includes a localoscillator for providing a radio frequency signal which, afteramplification, is transmitted, via a suitable transmitter antenna, as acontinuous wave (CW) radar signal. Energy reflected by an object in thepath of the transmitted CW radar signal is fed, via a suitable antenna,to a mixer circuit. Also fed to the mixer is a portion of the continuouswave signal produced by the local oscillator. The mixer thereby producesan intermediate frequency signal having as a frequency component thereofa frequency related to the Doppler velocity of the object producing thereflected energy.

As is also known in the art, in order to reduce the size of such mixercircuitry it is desirable to increase the operating frequency of thelocal oscillator. One application requires that the oscillator produce afrequency in the millimeter wavelength region. One type of transistoradapted in the millimeter wavelength region is a heterojunction bipolartransistor (HBT). Such transistor may include a semi-insulating galliumarsenide (i.e., III-V material) substrate, an N+ type conductivity GaAssubcollector layer, an N type conductivity GaAs collector layer, a P+type conductivity type base layer, an N type conductivity InGaP, orAlGaAs emitter layer and an N+ type conductivity type emitter contactlayer sequentially formed as a single crystal body. As is also known inthe art, it is desirable to form microwave circuitry as a monolithicmicrowave integrated circuit (MMIC) where both active devices, such astransistors, and passive devices, such as microstrip transmission lines,are also formed on the single crystal body. While such MMIC isdesirable, such may not be practical to manufacture because one type ofactive device may not be readily compatible, from a manufacturingaspect, with other types of devices. That is, the two types of devicesmay not be readily formed with sufficient common processing steps toproduce an MMIC which is economically practical. For example, while itis desirable to use Schottky diodes in the mixer circuitry discussedabove, Fabrication of the HBT oscillator of the mixer with sufficientprocessing steps common with the production of the Schottky diodes toresult in an economically process has not been described.

As is also known in the art, in order to minimize the conversionefficiency of the mixer circuit, the diode used therein should be DCbiased to some optimum operating point on its voltage-current (i.e.,V-I) characteristic curve. However, the DC bias point varies with thepower of the local oscillator signal. Therefore, because the power ofthe local oscillator varies with the operating temperature of theoscillator, the mixer will not operate with minimum conversion loss overthe operating temperature of the mixer circuitry.

SUMMARY OF THE INVENTION

In accordance with the present invention, mixer circuitry is providedhaving formed in a semiconductor body, an oscillator having aheterojunction bipolar transistor and, a mixer having a Schottky diode.The heterojunction bipolar transistor has a collector region formed inone portion of a doped layer of the semiconductor body and the Schottkydiode has a metal electrode in Schottky contact with another portion ofsuch doped layer.

With such an arrangement, mixer circuitry may be practicallymanufactured as a monolithic microwave integrated circuit.

In accordance with another feature of the invention, a mixer is providedcomprising a diode and a DC biasing circuit. The DC biasing circuitcomprises a constant current, for biasing such diode to predeterminedoperating point substantially invariant with power of an input signalfed to such mixer.

BRIEF DESCRIPTION OF THE DRAWING

Other features of the invention, as well as the invention itself, willbecome more readily apparent from the following detailed descriptionread together with the following drawings, in which:

FIG. 1 is a schematic block diagram of mixer circuitry according to theinvention;

FIG. 2 is a schematic diagram of a mixer used in the mixer circuitry ofFIG. 1;

FIG. 3 is a plan view of the mixer of FIG. 2;

FIG. 4 is a series of curves showing the relationship between mixerconversion loss and local oscillator signal power for various biasvoltages applied a mixer circuit;

FIG. 5 is a curve showing the relationship between local oscillatorpower and mixer conversion loss for the mixer of FIG. 2;

FIG. 6 is a diagrammatical cross sectional view of a semiconductor bodyadapted to have formed therein both a Schottky diode for the mixer ofFIG. 2 and an HBT for a local oscillator used in the mixer circuitry atone stage in the fabrication thereof;

FIG. 7 is a diagrammatical cross sectional view of the semiconductorbody of FIG. 6 after such body has been fabricated from an HBT used inthe mixer circuitry of FIG. 1;

FIG. 8 is a diagrammatical cross sectional view of the semiconductorbody of FIG. 6 after such body has been fabricated with a Schottky diodeused in the mixer of FIG. 2; and

FIG. 9 is a plan view of the Schottky diode of FIG. 8, the cross sectionof FIG. 8 being taken along line 8--8 in FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, mixer circuitry 10 is shown having a localoscillator 12 providing a local oscillator signal for a mixer 14. Themixer 14, shown in detail in FIG. 2, is also fed by a radio frequency(RF) signal to produce an intermediate frequency (IF) output signalhaving as a frequency component thereof the difference between thefrequency of the local oscillator signal, here 76.5 GHz, and thefrequency of the RF signal, here having a nominal frequency of 75.5 GHz.The oscillator 12 and mixer 14 are formed in a single crystalsemiconductor body, here GaAs body 16, as a monolithic microwaveintegrated circuit (MMIC).

Referring to FIG. 2, the mixer 14 is here a DC biased rat race mixerhaving a pair of Schottky diodes 20, 22. The anode of diode 20 iscoupled to the local oscillator signal through a quarter wavelengthtransmission line 24 and to the RF input signal through a quarterwavelength transmission line 26, as shown. The cathode of diode 22 iscoupled to the local oscillator signal through a pair of seriallyconnected half and quarter wavelength transmission lines 28, 30,respectively, and to the RF input signal through a quarter wavelengthtransmission line 32, as shown. A pair of DC blocking capacitors C1, C2is provided, as shown. Diode matching tuning stubs 34, 36, 38 and 40 andRF radial ground stubs 42, 44, 46, all arranged as shown, are alsoprovided. The IF signal is produced by mixer 14 by coupling the junction47 between the half wavelength transmission line 28 and the quarterwavelength transmission line 30 to output line 48 via half wavelengthtransmission line 49, as shown. A layout of the mixer 14 is shown inFIG. 3 with like elements being indicated by like numerical designation.

Referring both to FIGS. 2 and 3, it is noted that the cathode of diode20 is connected to a -terminal 60 and the cathode of diode 22 isconnected to a +terminal 62. A constant current source 66, shown in FIG.2, is connected to the terminals 60, 62 to provide a DC bias to thediodes 20, 22. The current, I, fed to the diodes 20, 22 by the constantcurrent source is selected to maximize the conversion efficiency of themixer 14 over the expected operating power level of the local oscillatorsignal.

More particularly, referring to FIG. 4, and assuming for a moment thatinstead of coupling constant current source 66 to terminals 60, 62, avoltage source were coupled to such terminals 60, 62. The relationshipbetween mixer conversion loss and local oscillator power is shown forunder such assumption for different diode bias voltages V_(B1) throughV_(B4) provided by the voltage supply. Is noted that for any biasvoltage the conversion loss increases with decreasing local oscillatorsignal power. Further, while a bias voltage of V_(B1) may provide theoptimum bias level for power levels greater than P₁, bias V_(B3)provides the optimum bias level for power levels less that P₂ andgreater than P₁, Thus, in order to optimize the conversion efficiency ofthe mixer, the DC bias provided by the assumed voltage source would haveto vary as a function of the local oscillator power.

We have found, however, that when operating the diode with the biasvoltages V_(B1) through V_(B4) over the range local oscillator powerfrom P₁ through P₄, i.e. at the optimum bias voltage level to minimizeconversion loss, substantially the same level of DC current passed tothe diode. Here, the constant current source 66 produces a current of 1milliamp. For a mixer with a local oscillator frequency of 76.5 GHz, anRF signal frequency of 75.5 GHz, and an RF input power level of -15 dBm,the conversion loss as a function of local oscillator power is shown inFIG. 5. It is noted that the mixer "turns on" (i.e., there is a sharpdrop in conversion loss to a suitable value) below 0 dBm of the localoscillator signal power. It is also noted that the conversion lossremains relatively flat for higher levels of oscillator signal power.

Referring now to FIG. 6, the semiconductor body 16 is shown at one stagein the fabrication of the MMIC. The body 16 includes: a semi-insulatinggallium arsenide (i.e., III-V material) substrate 70; an N+ typeconductivity GaAs subcollector layer 72; an N type conductivity GaAscollector layer 74; a P+ type conductivity type base layer 76, an N typeconductivity InGaP, or AlGaP, emitter layer 78 and an N+ typeconductivity type emitter contact layer 80 sequentially formed in amanner described in detail in copending patent applications Ser. No.08/744,025 filed Nov. 5, 1996, inventors: Elsa K. Tong et al. (Docket36756); and Ser. No. 08/740,339 Filed Nov. 11, 1996, inventors: Elsa K.Tong et al. (Docket 36757), assigned to the same assignee as the presentinvention, the subject matter thereof being incorporated herein byreference. A heterojunction bipolar transistor HBT 82 used in the localoscillator 12 (FIG. 1) is formed by subsequently processing the body 16,such bipolar transistor 82 being shown in FIG. 7. The HBT 82 includesbase contact electrode 90, an emitter contact electrode 92 and acollector contact electrode 94.

The Schottky diodes 20, 22 are formed as mesas in another portion of thebody 16, an exemplary one of such diodes 20, 22, here diode 20 beingshown in FIGS. 8 and 9. It is noted that the cathode contact electrode100 is formed during the same process step that the collector contactelectrode 94 (FIG. 7) is formed. Further, the emitter layer 78 (FIG. 7)is removed from the region of the body 16 where the diode 20 is to beformed; i.e., the region shown in FIG. 8. An opening 102 is formed in aportion of the underlying base layer 76 and into the upper portion ofthe exposed collector layer 74, as shown in FIG. 8. After the collectorcontacts 94 and cathode contacts 100 are formed during the sameprocessing step, an insulating layer, here a layer 106 of siliconnitride is deposited over the surface of the body 16. The siliconnitride layer 106 is patterned with the opening 102 to expose a portionof the collector layer 74. A suitable metal 90, here 1000 Å titanium,1000 Å platinum and 8000 Å gold, is deposited directly onto the exposedsurface of the collector layer 74 and processed to form a Schottkybarrier contact with the collector layer 74. The metal 110 serves as theanode of the diode 20. The cathode contact 100 is made through the topof the body 16 by etching down and contacting the very highly dopedsubcollector region 72 at during the same processing step used to formthe collector contact electrode 94. The doping level of the collectorlayer 94 is here in the range from 2×10¹⁶ atoms per cm² to 5×10¹⁶ atomsper cm² and the thickness of such layer 72 is here 0.5 micrometers. Suchdoping level and thickness for the collector layer 72 was found to bethe best compromise between diode 20 and HBT 82 (FIG. 7) performance.The area of each of the diodes 20, 22 was 15 square microns (i.e., eachdiode 20, 22 included three anode fingers 110₁ -110₃ and 4 cathodefingers 112₁ -112₄, as shown in FIG. 9 for diode 20). It is noted thatthe anode fingers 110₁ -110₃ and cathode fingers 112₁ -112₄ areconnected to anode and cathode contact pads 110, 106 as conventionalair-bridges.

Other embodiments are within the spirit and scope of the appendedclaims. For example, the HBT may be used in an amplifier circuit whichis formed in a common III-V body with the Schottky diode of a mixer. Inapplications where an amplifier is also to be formed with a mixer, theamplifier may include the HBT so that the amplifier HBT and the mixerSchottky diode are formed in different regions of the same II-V body.

What is claimed is:
 1. A mixer circuit, comprising:a diode; and a DCbiasing circuit, comprising:a constant current, for biasing such diodeto predetermined operating point substantially invariant with power ofan input signal fed to such mixer.